The present invention relates to a process for manufacturing catalysts for use in dehydrogenation reactions and, particularly, in the dehydrogenation of saturated or monoethylenic aliphatic hydrocarbons of low molecular weight (containing, for example, from 2 to 8 carbon atoms) as well as in the dehydrogenation of alkylaromatic hydrocarbons (such as ethylbenzene or diethylbenzenes) to vinyl aromatic hydrocarbons (such as styrene or divinylbenzene). It also relates to the catalysts obtained by this process and their use in dehydrogenation reactions.
It is known that, in the dehydrogenation of the above-mentioned hydrocarbons, the hydrocarbon is passed, preferably with an additional high proportion of steam (1 to 30 moles H.sub.2 O/mole hydrocaron), over a catalyst at an hourly rate by volume, expressed in relation with the liquid hydrocarbon, from 0.05 to 5, preferably from 0.1 to 1 volume (STP) per volume of catalyst and per hour and at a temperature of about 450.degree. to 750.degree. C.
Prior art catalysts have been described for dehydrogenating olefinic hydrocarbons, such as butenes (to butadiene) or alkylaromatic hydrocarbons, such as ethylbenzene (to styrene), these catalysts containing a major proportion by weight of iron oxide, a potassium compound (oxide or carbonate), vanadium oxide, optionally chromium oxide, as well as small proportion (from 0.01 to 10% by weight) of at least one additional oxide of a metal such as aluminium, cadmium, copper, magnesium, manganese, nickel, rare earth metals, uranium and zinc. Such catalysts are described in particular in the French Pat. No. 2,387,200 which substantially corresponds to the U.S. Pat. Nos. 4,143,083 and 4,152,300. In these patents, the rare earth metals are defined as being the metals of atomic number from 58 to 71 included, i.e. from cerium to lutetium. The rear earth metals used in the examples are cerium, praseodymium and neodymium.
As a general rule, the catalysts are prepared by admixing in suitable proportions compounds of the metals included in their composition. This operation consists more particularly of a malaxing in the presence of water, so as to form a paste which is then optionally shaped (for example by extrusion), dried and thermally activated.
The French Pat. No. 2,270,003, which corresponds substantially to the U.S. Pat. No. 4,134,858, discloses the use of a clayish material in the manufacture of a dehydrogenation catalyst, mainly based on iron, chromium and potassium oxides. During the thermal activation, at a convenient temperature, generally between 850.degree. and 1100.degree. C., the clayish material combines with potassium oxide to form a double aluminium and potassium silicate (more particularly kaliophyllite). This technique makes it possible in particular to decrease the filling density of the catalyst.
It has now been discovered that is was possible to improve the catalytic properties of catalysts containing, as oxides, iron, chromium and potassium, as well as a rare earth metal, by a two-phase process which will be described hereinafter.